Many electronic devices require a reference voltage to implement their design. The reference voltage may be used to control the electronic device or may, for example, be compared to another voltage. These uses require that the reference voltage remain stable. The challenge is to provide a reference voltage generator which gives a stable voltage despite temperature and power supply (voltage) variations, or others.
One type of device that is used to generate a reference voltage is a "bandgap" circuit. The bandgap circuit was originally developed for bi-polar technology. It has been modified for use with Complementary Metal Oxide Semiconductor ("CMOS") technology. Among the elements used to implement the modified bandgap circuit are transistors biased as diodes. This type of bias requires the P-N junctions of the transistors to be forward biased. This type of biasing is not well-suited for CMOS technology since any generation of substrate current may cause the bandgap circuit to latch-up. Manufacturers avoid this problem by using specially isolated wells in the semiconductor manufacture in order to collect the current.
Another reference voltage generator, as shown in FIG. 5, provides a reference voltage determined by the difference between the threshold voltages of transistors used in the device. Referring to FIG. 5, a transistor 40 has a threshold voltage V.sub.T1 that is less than the threshold voltage V.sub.T2 of transistor 42. V.sub.REF is calculated by the equation: EQU V.sub.REF =V.sub.T2 -V.sub.T1. (1)
For example, if V.sub.T1 =-1.6 V and V.sub.T2 =-0.6 V, then V.sub.REF =+1.0 V. In this example, both transistors are P-channel devices, and each has a respective threshold voltage.
However, most CMOS technologies readily provide P-channel MOS transistors on a chip with uniform, single V.sub.T. Extra processing steps, such as masking and implanting, are needed to fabricate a P-channel transistor with another V.sub.T. These extra steps add considerable expense to the fabrication of this second device and the resulting circuit.
It is the general object of this invention to overcome the above-listed problems.
Another object of the present invention is to allow the use of any standard CMOS or MOS processes, thereby to obviate extra or costly processing.
A further object of the present invention is to implement a reference voltage generator that works well at low voltages and despite wide voltage variations.
Still another object of the present invention is to provide a reference voltage generator that has low power consumption.
A salutary object of the present invention is to provide a reference generator which can be designed to have a positive, negative, or an approximately zero temperature coefficient.